G.A mispairs are one of the most common noncanonical structural motifs of RNA. The 1.9 A resolution crystal structure of the RNA 16-mer r(GCAGAGUUAAAUCUGC)2 has been determined with two isolated or nonadjacent G.A mispairs. The molecule crystallizes ...

G.A mispairs are one of the most common noncanonical structural motifs of RNA. The 1.9 A resolution crystal structure of the RNA 16-mer r(GCAGAGUUAAAUCUGC)2 has been determined with two isolated or nonadjacent G.A mispairs. The molecule crystallizes with one duplex in the asymmetric unit in space group R3 and unit cell dimensions a = b = c = 49.24 A and alpha = beta = gamma = 51.2 degrees. It is the longest known oligonucleotide duplex at this resolution and isomorphous to the 16-mer duplex with the C.A+ mispairs [Pan, et al., (1998) J. Mol. Biol. 283, 977-984]. The C.A+ mispair behaves like a wobble pair while the G.A+ does not. The G.A mispairs are protonated at N1 of the adenines as in the C.A+ mispairs, and two hydrogen bonds in the G(syn).A+(anti) conformation are formed. The syn guanine is stabilized by an intranucleotide hydrogen bond between the 2-amino and the 5'-phosphate groups. The G(syn).A+(anti) conformation can provide a different surface for recognition in the grooves compared to other G.A hydrogen bonding schemes. The major groove is widened between the two mispairs allowing access to ligands. One of the 3-fold axes is occupied by a sodium ion and a water molecule, while a second is occupied by another water molecule.